The present embodiments relate to a method for determining a radiotherapy treatment plan for radiotherapy treatment of an object to be irradiated and to a corresponding device. The present embodiments further relate to a method for determining a radiotherapy treatment plan for radiation treatment of a patient with multiple beams as part of particle therapy (i.e., multi-beam particle therapy).
In multi-beam particle therapy, which is also known as multiple field therapy, radiation delivered from a number of beams with different angles of incidence is combined together into a conformant dose for distribution over a target volume. The particle fluence profiles of the beams will normally be determined by an automatic optimization method that operates on the basis of medical targets that are defined by a human user. These targets are usually expressed as conditions in relation to the resulting dose distribution. There are two principle approaches to specifying the dose conditions: either the user specifies the desired part doses of each beam separately, such that, for example, they are homogeneous over the target (single beam optimization, SBO), or the user specifies the desired sum dose of all beams (intensity-modulated particle therapy, IMPT) over the target. In IMPT, no conditions are imposed on the single beam doses and the single beam doses may thus be very inhomogeneous. Both methods, SBO and IMPT, which are shown below with reference to FIGS. 1-8, have their advantages and disadvantages.
FIG. 1 shows what is referred to as a dose-volume histogram, in which a radiation dose, measured in gray (Gy), is plotted on the x-axis and a tissue volume proportion, measured in terms of a percentage, is plotted on the y-axis. Graphs 1-3 each specify, for an area of tissue, a percentage of the respective tissue (y-axis) that receives a radiation dose based on the x-axis value or a value greater than the x-axis value. Such a dose-volume histogram (DVH) may, for example, be determined by a simulation for predetermined irradiation parameters before radiation therapy. In FIG. 1, graphs 1 and 2 show examples of a dose-volume histogram for sensitive areas of tissue or organs (i.e., for areas of tissue or organs which lie in the path of the radiation during radiation therapy but are to be irradiated as little as possible since the tissue or organs do not represent the target region of the radiation therapy). The objective is thus for these graphs to lie as far to the left as possible in the dose-volume histogram (i.e., to receive as little total radiation as possible and to receive the lowest possible maximum radiation). Graph 3, by contrast, shows the dose-volume histogram for a target region, such as, for example, a tumor region, which is also referred to as the Planning Target Volume (PTV). In order to obtain a homogenous and high irradiation of the target region, graph 3 in the dose-volume histogram of the target region should, if possible, fall away in steps at the desired target dose from 100% to 0%. With an ideal step, this would mean that 100% of the target region receives the desired radiation dose. In the dose-volume histogram of FIG. 1, the graph 3 deviates slightly from an ideal step function as a part of the target volume is irradiated with a radiation dose that is lower than the desired radiation dose of 1 Gy and another comparatively lower proportion of the target volume is irradiated with a radiation dose that is higher than the desired radiation dose.
FIG. 1 shows the dose-volume histogram which is created based on the IMPT method. The user pre-specifies 1 Gy as the desired radiation dose for the target region. In the example shown, the total dose for the target region is composed of two beams, which, for example, act on the target region from opposing directions. FIG. 2 shows the total dose over the irradiated region, with a size of the object or region to be irradiated in the patient plotted on the x-axis and the radiation dose, in gray (Gy), achieved in that portion of the object or region plotted on the y-axis. In the desired planning target volume (PTV) of, for example, 60 mm-120 mm, a comparatively homogeneous total dose of a good 1 Gy is achieved. FIGS. 3 and 4 show how the total dose of FIG. 2 is composed of the two beams. FIG. 3 shows the radiation dose as a result of a first beam, and FIG. 4 shows the radiation dose as a result of a second beam. A strong inhomogeneity of the individual beams in the planning target volume (PTV) is evident here. The first beam produces a very high radiation dose in the region between 100 and 120 mm, and the second beam produces a very high radiation dose in the region from 60 to 80 mm. This type of high beam inhomogeneity (i.e. widely differing doses of radiation resulting from individual beams), is, however, undesirable since the danger arises that overall, an inhomogeneous irradiation of the planning target volume results if the object to be irradiated is repositioned between the irradiation with the first beam and the irradiation with the second beam but the repositioning is not performed with sufficient accuracy. Repositioning during the treatment is only one possible error source. In particle radiation therapy, a globally incorrect positioning may also cause a displacement of the dose distributions relative to one another. Another error source is the product of patient geometry changes that occur when, for example, a patient loses weight between planning and treatment. In addition, there is the danger that sensitive tissue, which is to be irradiated as little as possible, receives an undesirably high radiation dose, especially after an insufficiently exact repositioning of the object to be irradiated. Although in principle the conventional IMPT produces a good and homogeneous irradiation of the target volume, as is evident from FIG. 1 and FIG. 2, and protects the sensitive healthy tissue during this process, the conventional IMPT is very susceptible to positioning errors and patient movements during the irradiation.
FIG. 5 shows a dose-volume histogram which is created with the aid of the SBO method. In FIG. 5, graphs 1 and 2 show radiation doses for sensitive tissue in the beam path, and graph 3 shows the radiation dose for the planning target volume (PTV). As is evident from FIG. 5, especially when compared with FIG. 1, the total coverage of the target volume with an even radiation dose is worse than with the IMPT method of FIG. 1. FIG. 6 shows the entire dose profile which is created by two opposing beams with the beam profiles of FIGS. 7 and 8. By comparing FIGS. 7 and 8 with FIGS. 3 and 4, it is shown that in the SBO method a higher single beam homogeneity may be achieved, so that the SBO method is more robust relative to planning and beam supply uncertainties or positional inaccuracies that occur while an object is repositioned between application of the first beam and of the second beam. In addition, homogeneous single doses are more robust relative to (initial) positioning errors.
Methods for optimizing either the IMPT method or the SPO method are thus known in the prior art. For example, Martin Soukup et al., in “Study of Robustness of IMPT and IMRT for Prostate Cancer against Organ Movement” (Int J Radiat Oncol Biol Phys. 75(3):941-9 (2009)), propose a method for initial beam weighting for an IMPT method in order to obtain better start conditions for the optimization. Furthermore, F. Albertini et al., in “Degeneracy and Robustness of IMPT Plans in the Treatment of Skull-Base Chordomas” (Med. Phys. Volume 34, Issue 6, pp. 2431-2431 (2007)), propose that the IMPT method only be used as an entry point and, thus, that only a part of the total dose be delivered via the IMPT method and the majority of the dose be delivered via beams which have been optimized with the SBO method.